SERS nanotags and other optically detectable nanoparticles have proved useful for marking objects for identification and tracking. SERS nanotags are nanoparticulate optical detection tags which function through surface enhanced Raman scattering (SERS). SERS is a laser-based optical spectroscopy that, for molecules or other materials, generates a fingerprint-like vibrational spectrum with features that are much narrower than typical fluorescence. SERS nanotags and other types of optical detection nanotags can be fabricated in various sizes, shapes and with some control over selected configuration elements. For example, a quantity of nanotags may be fabricated generally having a metal nanoparticle core and semiconductor shell. Within the total quantity of particles fabricated, some examples might have a single particle core and others a double particle core or a multiple particle core. Some individual configurations of generally related particle types have been observed to have enhanced performance characteristics. For example a SERs nanotag with two or more cores is typically observed to be spectroscopically brighter than a single core particle fabricated from the same materials or in the same batch. It would thus be desirable, for example, to sort the multiple core particles from the single core particles in a given batch.
In addition, it is exceptionally difficult or impossible to fabricate completely homogeneous populations of a selected particle configuration. For example, if SERS nanotags with two cores and a single shell are desired, fabrication parameters can be controlled to favor the creation of the desired configuration, but all known fabrication techniques will produce substantial quantities of tags with single cores as well. Thus, sorting becomes desirable, but it is exceptionally difficult to sort nanoparticles using conventional filtering techniques because of the nanoscale particle size.
In particular, known separation techniques such as filtering, centrifuging, or more advanced separation techniques are often difficult or impossible to implement with suspensions of nanoparticles. Known separation techniques can be particularly difficult to implement when the suspension contains more than one distinguishable nanoparticle type, size, shape or composition and it is desired to separate the different types of nanoparticles from each other. The embodiments disclosed herein are directed toward overcoming one or more of the problems noted above.